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Messages - Enzo

#1981
The Newcomer's Forum / Re: Noisy Heathkit TA-16
December 18, 2009, 11:27:05 PM
Transistors go bad whenever they want to, there is no timetable.
#1982
Think of it like this:  You meant to buy 5% resistors and got 1% resistors by mistake.


All it means is your probe can be used in more demanding places than your scope.
#1983
Doesn;t matter what order an RC is in, but in this case, the cap is in series but the resistor is in shunt. 

A schematic that is close should be enough to do the repair with.  I don;t even know if Fender published documentation for those.  They are "replace only" under warranty, they will not allow us to repair them under warranty.  No, I am wrong, it was the only Squire guitar amp schematic in the service files, but there it is.  Please note that the clumsy scan is right off the factory service disc, not my fault.

SHould be what you need.
#1984
The Newcomer's Forum / Re: Crate GX-212
November 28, 2009, 11:28:38 AM
I'm in the insert jack camp.

You have any effects loop jacks?  Next time this happens, plug a spare cord from FX send to FX return.  If that restores the sound, then the return jack needs service.  Likewise if there are a pair of jacks Preamp Out and Powr amp In, connect them together the same way.

In fact, if they are the problem, even when the amp is working, many times just probing into the return jack or powr amp jack with some plug will trigger the problem.

What I do inside the amp is measure the resistance across the tip cutout contacts on those jacks.  if it measures any more than half an ohm, the contacts need cleaning.


If it is not those, one thing I would watch for is a collapsing 15v power supply rail.  A failing zener most likely.
#1985
" The crack is faint , I don't know if it has broken the circuit ."

But that is a pretty important question now isn;t it?  Poke the leads from your ohm meter to points on either side of the possible brweak and measure for continuity.  That will tell you.

The "stress" of being soldered to the board didn;t break the board.  Someone stepping on their guitar cord more likely was what happened.  Or posssibly the amp fell on its face while something was plugged into the jack.

As to the problem, it is important that all areas of the circuit needing a ground connection get one.  If the board is cracked, a ground trace could be severed leaving some part of the circuit floating.  Of course all the traces need to be there, they don;t add extra ones.  So while grounds come to mind as they cruise around the edge of hte board, ther are other things there as well.
#1986
It is not voltage that kills tubes, it is dissipation.

If you ran them at 400v, you could bias them to 5ma, but that would only be 2 watts of dissippation.  And I suspect they would indeed last a long time at that level and sound crappy.
#1987
The Newcomer's Forum / Re: newbie capacitor questions
November 19, 2009, 03:35:07 AM
Yeah, generally  capacitance (in uf)/ voltage  so 50uf/50v or 6000uf/50v

SOmetimes you see a dual section cap, like two 50uf 500v sections in one can.  That could be written as 50/50 500v.  But that should be clear from the context it is in.


Many schematics have a note saying all caps are in uf unless noted, or even all are in pf unless noted.  SO all the caps would have plain numbers for pf and if they were uf, it would say so next to the part.

Polarity indicates the cap is either electrolytic or tantalum.  In my opinion, if you are learning, stay away from tantalum.  They work fine, but are extremely intolerant of things like reverse voltage and over voltage.


You can use most any type cap, but a 1uf electrolytic is the size of a resistor, while a 1uf film cap is many times larger.  And you just won;t find 470uf film caps.  They'd be huge though.

Mostly the caps I use in tube amps and the caps I use in solid state amps come from different drawers.


In my opinion, the differences are more subtle between the various film types.  learn about circuits first, and worry about the subtleties later.















#1988
Amplifier Discussion / Re: Loud humming problems
November 10, 2009, 09:16:32 PM
I'm with RG here.  You probably blew the output stages of the amp.  When you flip it on, look at the speaker cone.   Does it move one direction and stay there?  That would mean DC on the output.  If your practice amps are common, there will be a five-legged power amp IC on the heat sink, and it will have died.  Number like TDA2040 on it.
#1989
I can't play sounds at all presently anyway.  perhaps you can describe what it is doing?


And isolate the problem.   Does the sound come out with the controls all at zero?  In fact do ANY of the controls have ANY effect on this sound?
#1990
The Newcomer's Forum / Re: Troubleshooting help needed
November 08, 2009, 10:34:59 PM
I find that the op amps themselves are a lot more likely to fail than the resistors and caps around them.  And since they sell for about three for a dollar at Mouser, I'd have a new one in that spot by now to see if it were the problem or not.

Looking from a troubleshooting perspective, op amps are pretty much little functional blocks.  You don't really need to understand all that much about how they work.  Signal goes in one end, comes out the other.  If there is nothing coming out, my first suspect is the IC.


Not suggesting not to learn how they work, but thinking in terms of just getting it fixed.
#1991
A volume control in the FX loop.   Or something like an EQ pedal in the loop set for level reduction.


MAny, if not most, amps have two input jacks side by side.  One of those jacks is a little quieter than the other.  All the Fender amps come to mind.  The difference in loudness between them is 6db - 6 decibels.   Reducing 30 watts to 15 watts will reduce the loudness only 3db - 3 decibels.

To reduce the loudness by half, you need a powr reduction of 10 times.  In other words, 10 watts is half as loud as 100 watts.

SO a 3 watt amp will be half as loud as a 30 watt amp.
#1992
The Newcomer's Forum / Re: amplifier died...
November 03, 2009, 11:05:37 PM
Unplugging from the wall and turning the power switch off are the same thing to the amp - the mains power stops.

Please do connect your speakers and the same speasker cord to some other amp and test them.

If your amp is now silent qand the speakers good, then I'd look in the amp for blown fuses.  Does your model have a speaker fuse on the rear panel?  if not, look inside.

What amp do you have?   A 200 watt model?   Turning the volume control to half does not limit power.
#1993
If you just want to find the signal levels through the amp, apply a sine wave and use your AC volt meter.  If you have a "true RMS" meter great.  I find that my basic FLuke meters read AC pretty well up to at least 1kHz.  Certainly at 40Hz it is OK.

Now that tells you little about tone or frequency response, but your question was about levels.

The use of a steady test tone yields consistent results.   It is darn difficult to get steady test levels from a guitar.

A scope is a natural for this, you can look at the vertical range of the signal and frequency doesn;t much matter.

Your guitar amp is not hifi, it is not flat in response, so the note or frequency of your test signal makes a difference in signal level.
#1994
Tubes and Hybrids / Re: Drive channel is too dirty!
November 02, 2009, 11:57:47 PM
Too large to post here, but the whole thing is on Schematic Heaven

http://www.schematicheaven.com/fenderamps/champ_25_se.pdf

I don;t expect a lot of clean from a dirt channel to start with, and frankly, much as I like Fender amps and their clean channel, I have never hear a Fender overdrive channel I thought was worth a damn.


Sure there are 7 diodes in that one stage, but step back and look.  The ENTIRE dirt channel is ONE little dual op amp, and all those diodes are around just one side of it.   How sophisticated a tone can we expect from one op amp stage with diodes?  In my own opinion, you can mess with the diodes, but that will only change that part of the tone contributed by the diodes themselves.  Certainly I am not suggesting you don;t try though.

Does it help to set the gain lower and the volume higher?  As in giving the diode stage less to clip on?  Or does it sound crappy no matter what?
#1995
I think we might be discussing this same amp on another forum.   I suggested a bad op amp in that channel.  And from that era, they are probably already in sockets.